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A simple method to "estimate" the weight of an oddly shaped object, such as your rock, could go like this:

Measure the cross section of the object and remember to always pass the ruler or tape across the center of the face you're measuring. Imagine the rock with a front, a side, and a top and take several measurements of each as an example, the width, height, and both diagonals of each face.

Add all those numbers together and divide the sum by the number of measurements you took. The resulting number is the "average" diameter of your rock. I'll use your measurements but remember that the more you take, the more accurate the estimate will be.

(39 + 21 + 51) / 3 = 37

The average diameter is 37". Now a simplified volume formula as this is, after all, an estimate.

3.14 * 37 * 37 * 37 / 6 = 26508.403

Converted to cubic feet is

26508.403 / 1728 = 15.34051

The volume of the rock is 15.34051 cubic feet then and the average weight of "many" types of rock is in the 160 to 170 pound per cubic foot range so: